Soma is the name of a god worshipped by the Aryan Indians in Vedic times, as well as of a certain plant, and its juice used for the preparation of an intoxicating drink. The word is most probably derived from the root su (cf. śu, 'to press out,' 'distil,' and thus originally meant 'extract.' The Soma cult, with its copious libations and potations of the sacred beverage, is not confined to India, but was already a prominent feature of the religious system of the Indo-Iranians; the old Persian haoma sharing all the characteristics and uses of the Indian soma. In his divine form Soma is conceived of as a powerful deity of a fiery and luminous nature, the inspirer of heroic deeds of arms, as well as of the flights of fancy and song, the bestower of health, long life, and even immortality. Indeed, in accordance with his physical origin, this fiery deity is at the same time the amrita (ambrosia), or the drink of immortality, alike for gods and men. At the Soma sacrifice, therefore, not only are libations of Soma juice made to the principal deities, but the sacrificer himself and the priests freely partake of the potent liquor. Among the celestials it is especially the god Indra, the Jupiter pluvius of the Vedic pantheon, with whom Soma stands in close connection; the ever welcome draughts of the fiery juice supplying him with the requisite strength and nerve for battling with the demons of drought and darkness. In the later Vedic literature Soma appears completely identified with the moon; this luminary in its varying phases being considered as the vessel containing the celestial supply of Soma juice on which the gods subsist, and which consequently requires periodical replenishment. Whilst this notion was formerly regarded as merely a secondary development of Indian mythology, recent research suggests that the identity of Soma with the moon may after all hold good for the whole of the Vedic times, if not also for the Indo-Persian period. The conception of a heavenly receptacle of the Soma juice forms, however, only one part of the Vedic Soma myth, which, indeed, pictures the god in all the various forms which the earthly Soma undergoes, from the time when the golden-stalked plant is brought down from its mountain-home till the final consummation of the Soma eup, as the 'supreme offering.' For his regeneration Soma descends periodically to the earth, either in the shape of the heavenly plant carried off by an eagle (probably the lightning) from his jealous demon keepers, or in the form of the fiery germ received by the cows or waters of heaven, and carried down by them to the earth, there to be taken up and matured by the plants springing up luxuriantly after the rains. The question as to what particular plant may claim to be regarded as the true Soma or 'moon-plant' is surrounded with difficulties; probably, however, the stems of several varieties of Asclepiads, akin to the common milk-weed, such as Asclepias acida, Sarcostemma brevistigma and viminale, and Periplocus aphylla, all of them containing a rich, milky juice, were used for this purpose. The Brahmans of the Deccan, as well as the Parsees or fire-worshippers of Kerman and Yezd in Persia, and those in Bombay, make use to this day of different species of this genus. The Soma sacrifice, as practised in the later Vedic times, is surrounded by a complicated ceremonial of great solemnity. Sixteen priests officiate at its performance. A feigned purchase of Soma plants for a cow, at the conclusion of which the low-caste vendor is beaten off the sacrificial ground with sticks, is apparently a symbolical representation of the acquisition of the Soma by the gods from the demons. The bundle of Soma plants is henceforth treated as the veritable King Soma: he is driven in a car in state to the sacrificial hall, where he is placed upon a throne-seat, a guest-meal being then provided for him, and a solemn covenant concluded between him and the sacrificer. The Soma juice is subsequently obtained by means of two pressings; and the libations are made at three different services, the morning, mid-day, and evening service. The ninth book of the Rigveda-sankhitā consists of over a hundred hymns composed for the special purpose of being recited in honour of Soma pavamāna—i.e. of the Soma juice, whilst fermenting and 'clarifying.' Animal offerings form a necessary part of the Soma sacrifice.
For further details, see Eggeling, Translation of the Satapatha Brāhmaṇa, vols. ii. and iii. (in the Clarendon Press series of Sacred Books of the East). On the divine personality of Soma, cf. J. Muir, Original Sanskrit Texts, vol. v.